The present invention relates to a method of and an apparatus for thickening lime sludge with a drum filter according to which lime sludge is thickened on a filtering layer formed of the lime sludge on the surface of a filter drum partly immersed in a vat containing lime sludge.
Conventionally, a lime sludge having a dry-solids content of approximately 25% is thickened to a dry-solids content of 70-85% with a drum vacuum filter, prior to supplying the lime sludge to a lime sludge reburning kiln. At the beginning of the filtering process, a precoat filter layer of the lime sludge is formed on the surface of the filter drum, the thickness of the layer normally being about 10-20 mm. A scraper is used for scraping off the dried lime sludge from the surface of the precoat. The dried lime sludge falls onto a conveyor and further onto a feed screw which introduces the dried lime sludge into a lime sludge reburning kiln.
The surface layer of the precoat becomes clogged while the filtering proceeds and it must be removed from time to time. The method normally used is to move an automatic scraper gradually inwards according to a predetermined program whereby the clogged surface layer is removed. After the scraper has moved in several steps closer to the drum the whole precoat is removed and a new precoat formed.
All the movements of the scraper described above, and the removal of the precoat from the surface of the drum, result in disturbances in the operation of the lime sludge reburning kiln. After the scraper has moved, the filtering of the lime sludge is more efficient and the lime sludge volume larger for a while. Usually also the dry-solids content of the lime sludge is higher after a movement of the scraper, which result in a change in the operation of the lime sludge reburning kiln. When the whole precoat is renewed the supply of lime sludge to the kiln is stopped for a while which causes a major disturbance in the kiln. Typically, the precoat is replaced once a shift or three times during twenty-four hours. During a longer period of time, the filter cloth becomes so badly clogged that it must be washed either with an acid or with a high-pressure washer. This operation also causes a disturbance, and subsequent reduction in production, of the lime sludge reburning kiln.
A characteristic feature of the method of the present invention resides in the continuous removal of a strip of lime sludge at a time from the filtering layer by means of a liquid jet, preferably reciprocating continuously in the longitudinal direction of the drum, parallel to its axis of rotation.
A characteristic feature of the apparatus of the invention is that at least one nozzle is provided under the scraper and continuously reciprocable in the longitudinal direction of the drum to effect sludge removal.
High-pressure nozzles (e.g. two) are provided under the scraper, mounted in such a way that one of the nozzles breaks the precoat and drops it into the vat and the other nozzle washes the filter cloth clean. The pressure of the water issuing from the nozzles is approx. 50-100 bar which gives a very good cleaning effect. Each nozzle spreads the jet in such a way that the precoat is removed from a strip of about 10-50 mm in width of the drum, and the same area of filter cloth is washed at that time.
The washing nozzles are installed in a rack designed for displacing the nozzles in the lateral direction. The nozzles are continuously reciprocated in the longitudinal direction of extension of the drum which continuously renews the precoat and washes the filter cloth.
Advantages of continuous high-pressure water scraping are: (1) The dry-solids content of the lime sludge from the lime sludge filter to the lime sludge reburning kiln is maintained continuously uniform, allowing constant operation of the kiln. (2) A change of precoat is not necessary as the precoat is renewed continuously a strip at a time. Disturbances in the reburning kiln due to a change of precoat are thus avoided. (3) There is no need to interrupt filtering for washing of the filter cloth.
It is the primary object of the present invention to provide an improved method and an apparatus for thickening lime sludge.